It is known in the art of cement manufacture that combustion ash, particularly bituminous coal fly ash, is a useful additive in the making of cements, by virtue of its pozzolanic properties. A pozzolan is a material which, when combined with calcium hydroxide released by cement (e.g. Portland cement) during its hydration with water, exhibits cementitious properties in the presence of water. Pozzolanic ash, such as class F bituminous fly ash, can be used as a component of blended cements if it meets certain quality specifications (e.g. ASTM C618).
The technical benefits to be gained from addition of such materials to blended cements include reduced use of Portland cement per unit volume of mortar or concrete, increased strength and reduced water permeability of the mortar or concrete. Economic benefits arise from the reduction in the amount of Portland cement used, the higher quality of the mortar or concrete produced and the minimization of coal fly ash sent to landfill.
The value of bituminous coal fly ash as a pozzolan is enhanced by reducing its particle size, which increases the surface area and reactivity of silica and siliceous materials towards the free calcium hydroxide generated by hydration of Portland cement, reduces water porosity and increases the compressive strength of the resulting concrete. Reducing the ammonia content of bituminous fly ash also increases its value as a pozzolan since little or no gaseous ammonia is released when the ammonia-depleted ash is added to cement clinker and water to make blended cement, mortar or concrete.
It is also known that the reduction of overall particle size distribution of aluminosilicate raw materials for cement kilns, including bituminous fly ash, enhances cement kiln throughput. Bhatty et al., in “Utilization of Discarded Fly Ash as a Raw Material in the Production of Portland Cement,” Journal of ASTM International, Vol. 3, No. 10 (2006), partially attributed a 9.7% increase in cement clinker production to lower particle size of coal fly ash as compared to conventional shale aluminosilicate input.
However, the high mercury content of bituminous fly ash relative to shale or clay dramatically reduces its acceptance as a cement kiln raw material. The only cement kiln raw material having a mean mercury content higher than coal fly ash is recycled cement kiln dust: Portland Cement Association, PC R&D Serial No. 2888, (2006). The high mercury content, in view of regulations governing mercury emissions from cement kiln operations, is recognized as an impediment to combustion ash usage in cement manufacture: Daniel Crowley, “Cement Kiln Mercury Reduction Strategies A Case Study in Materials Management,” Cement Industry Technical Conference, (2010).
Accordingly, there exists a need for practical, economical, large-scale processes that can upgrade combustion ash, especially bituminous coal fly ash, by reducing its mercury content.